Extruded thermoplastic pipe is often used to carry potable water and other liquids at hydrostatic pressures in the range of 160 to 400 pounds per square inch. In order to determine the ability of such extruded plastic pipe to withstand, for long periods, this relatively high internal hydrostatic pressure, it is necessary to test samples of such pipe at even higher pressures. It is also necessary to determine the ability of the pipe to withstand without rupture certain overpressure conditions, such as those conditions resulting from a surge of water pressure within the pipe.
In the past these tests have been performed by connecting a sample of such pipe to a source of water under pressure. The amount of pressure to which the pipe was to be subjected was monitored by an operator who observed a water pressure gauge. The internal pressure in the pipe sample was increased slowly by manually operating a pressure control valve connected between the water filled pipe and the water pressure source. A certain standardized test used to determine the ability of the pipe to withstand hydrostatic pressure without bursting calls for the internal pressure to be increase from 0 to the maximum pressure of the test within 60 seconds. Thus, in order to be a fair test the operator must being the pressure from 0 to the highest test pressure slowly and uniformly within that 60 second period. During this period, the sample of plastic pipe would yield both elastically as well as plastically in response to this increasing pressure. In doing so the rate in which the water entered the test sample would tend to change erratically. For example, as the yield point was reached the rate of water intake into the sample would suddenly increase. This sudden intake in water resulted in a pressure decrease within the pipe sample followed almost immediately by a sudden pressure increase, either as a result of the operator increasing the flow of pressurized water into the sample or the tendency of the sample itself to suddenly reduce its instantaneous rate of strain. The resulting fluctuations in the pressure application produced unpredictable and unrepeatable dynamic forces on the pipe sample which affected the validity and accuracy of the intended hydrostatic test procedure.